Obtaining hydraulic products or products with hydraulic potential from CO2 emission free raw materials meets with problems being inherent to their chemistry and mineral contents being adverse for obtaining a binder.
It is meant under “adverse” that the resulting product will not be useful, neither alone, nor possibly mixed with Portland cement as it will not give the strength performance required from the standards or as it will generate swelling and structural destruction problems. This is the case for LD slag (coming from solidified and ground steel industry slag).
LD slag is by-products from refining of hematite cast irons (cast irons with low phosphorus content) through the oxygen blowing process. Such a material has a high iron and lime content with a mean mineralogical composition resulting from the assembly formed with dicalcium silicates, calcium ferrite and metal oxides, so that the mean chemical composition of the main components is as follows:
Compound% weightCaO50SiO213Al2O33MgO6Iron oxides28Free ironUp to 20Free CaOUp to 10
Using LD slag in the form of granulates for concrete or road building for producing bitumen top layers and foundation layers is limited by the presence of free lime subject to create road or concrete expansions.
Converting LD slag into hydraulic binder also arouses quite a lot of interest.
Patent FR-2,546,530 discloses a treatment for steel industry slag with a view to their use as cement.
The treatment being disclosed in said patent consists in adding to the liquid slag a compound (bauxite) supplying alumina and a compound (aluminium) forming alumina, supplying the amount of heat required for dissolving the compounds in the slag and subjecting the slag to an oxygen mixing.
The amount of the compound(s) being able to form the alumina to be added to the slag is such that the treated slag contains 5 to 25% in weight of alumina.
Although Patent FR-2,546,530 discloses that the thus treated slag can be used as a hydraulic binder, including for producing cement, such a treatment does not make it possible to obtain a hydraulic binder as such, being able to entirely replace Portland cement.
Patent DE-2,611,889 discloses the oxidative treatment of residues from the steel industry through addition of lime followed with a very fast cooling step for obtaining a granulate with which 3 to 8% in weight of gypsum is ground.
The treated metallurgical residues primarily comprise furnace slag.
More precisely, such a process discloses a process for converting 60 to 90% of metallurgical scraps through addition of 10 to 40% in weight of lime. However, the thus treated scraps consist in 35% steel industry slag, mixed with 48% furnace slag and 17% other steel manufacturing scraps. Only approximately 20 to 32% of steel industry slag is actually treated.
The LD slag treated using such a process have a relatively low CaO content (<45% in weight) and a high Fe2O3 content (>30% in weight). Moreover, such a treatment occurs at a high temperature ranging from 1,600 to 1,750° C., preferably from 1,650 to 1,700° C. and requires an additional fuel to be used, such as coke.
The Applicant knows an oxidative treatment for steel industry slag, comprising the steps of adding to the slag an alumina source and a lime source and cooling them adequately for obtaining a treated slag having a mineral composition meeting one of the following compositions:
(a) an amorphous glassy phase;
(b) a first phase assembly (1) consisting, in % weight, in 10-40 CA, 20-50 C2AS, 30-50 C6AF2 and 10-30 C2S;
(c) a second phase assembly (2) consisting, in % weight, in 20-40 C2F, 10-30 C2AS, 20-50 C6AF2 and 10-40 C2S; and
(d) a mixture of an amorphous glassy phase and of the first or the second phase assembly.
According to the conventional cement manufacturers' expressions, it should be reminded that:
C=CaO
A=Al2O3 
S=SiO2 
F=Fe2O3 
P=PO4 
The above-mentioned phases are not pure compounds, but are likely to contain, in a solid solution, impurities such as iron, magnesia (MgO), phosphorus (P2O5), sulphur, etc.
In addition to the fact that such a process requires using controlled cooling steps, the amounts of alumina and lime sources to be added are important, generally for 25% in weight or more for the alumina source and for 40% in weight or more for the lime source.